This application claims the right of priority under 35 U.S.C. xc2xa7 119 based on Japanese Patent Application No. JP 2002-106686 which is hereby incorporated by reference herein in its entirety as if fully set forth herein.
1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a laser beam printer, or the like. An image forming apparatus is suited particularly to an electrophotography system, and include, for example, an electrophotographic copier, an electrophotographic printer (for example, an LED printer, a laser beam printer, or the like), an electrophotographic facsimile, and so on.
2. Description of the Related Art
(Introduction of a Process Cartridge)
In image forming apparatuses of an electrophotography system such as a copying machine, a laser beam printer, or the like, a photosensitive member surface is first made uniform in electric potential, light corresponding to image information is irradiated on the photosensitive member to form a latent image (distribution of electric potential on the photosensitive member surface, corresponding to image information), a developer (toner) being a recording material is supplied onto the latent image by a developing means to make an image visible, and the image is transferred to a recording sheet from the photosensitive member to form an image on the recording sheet.
A toner container being a toner storage section is connected to the developing means and toner is consumed while images are formed. Generally, a toner container, a developing means, a photosensitive member, and a charging means are in many cases integrally constituted as process cartridge, and a user exchanges the process cartridge for a new one when toner runs out, whereby it becomes possible to form images again.
(Introduction of a Residual Detecting Means)
In order to know at any time how much toner for formation of images remains in a process cartridge, a developer residual quantity detecting means capable of successively detecting a level of toner residual quantity is in some cases provided in a process cartridge, or a body of an image forming apparatus
(System of Measurement of an Electrostatic Capacity Among Electrodes)
A method of measuring an electrostatic capacity between electrodes of a plurality of electrode members as developer residual quantity detecting means, in which an additional circuit is comparatively simple and which is high in accuracy of detection, is disclosed in, for example, JP-A-2000-250380, JP-A-11-272060, and so on.
Such residual quantity detection system makes use of a phenomenon that an electrostatic capacity between electrodes varies according to a quantity of an insulating toner present between the both. When toner fills between the both, the electrostatic capacity is high. And as toner decreases, an air increases in amount between the both and the electrostatic capacity decreases.
Accordingly, an electrostatic capacity between the both and a toner residual quantity are beforehand related to each other, and then a level of toner residual quantity can be detected by measurement of electrostatic capacity. Such electrostatic capacity can be measured by applying a predetermined alternating bias for detection of residual quantity between electrodes and detecting an electric current value and a voltage value induced at that time.
The predetermined alternating bias for detection of residual quantity is in many cases common to a developing bias. That is, such toner residual quantity level detection system detects a level of toner residual quantity at a timing such as formation of image, or the like, in which a developing bias is applied to a developing roller.
(Relationship Among Process Speed, Charging Frequency, and Developing Frequency)
Subsequently, an explanation will be given to a relationship among a charging bias (charging voltage) applied to a charging device in order to make a surface of a photosensitive member uniform in electric potential, a developing bias (developing voltage) applied to a developing device in order to fly a developer so that a developer (toner) is adhered to a latent image formed on the uniformly charged photosensitive member surface by exposure, and a process speed (print speed). In addition, the larger a process speed, the larger a peripheral speed (moving speed of a photosensitive member surface) of a photosensitive drum.
With a configuration using a contact type charging device making use of a charging bias with a direct current component and an alternating component as superimposed and a non-contact type developing device making use of a developing bias with a direct current component and an alternating component as superimposed, an appropriate value of a frequency of the charging bias generally increases with a process speed (however, in the case where image resolution is the same). Meanwhile, a frequency of the developing bias involves an appropriate range in developing characteristics, irrespective of a print speed.
Conventionally, a frequency of the developing bias is set to a large value relative to a frequency of the charging bias while there has been caused a need of approximating a frequency of the developing bias and the frequency of the charging bias to each other since the frequency of the charging bias is increased due to an increase in process speed in recent years.
(Problems caused by Approximation of a Frequency of the Charging Bias and a Frequency of the Developing Bias to each other, and a Method of Avoiding the Same)
When a frequency of the charging bias approximates to a frequency of the developing bias, cyclic density unevenness appears, in some cases, on an image depending upon the relationship between frequencies and a process speed. This will be described later in detail.
In order to avoid this, it suffices that, for example, a frequency of a charging bias be made equal to frequency of a developing bias.
As described above, by making a frequency of a charging bias equal to frequency of a developing bias, it is possible to solve the problem involved in cyclic density unevenness.
In an apparatus, in which a plurality of electrode members are arranged and a developing bias is applied between electrodes thereof for detection of a developer residual quantity, however, there is caused a new problem that by making a frequency of a charging bias and a frequency of a developing bias the same, a detected value of a developer residual quantity shifts due to a phase difference between the two frequencies.
In particular, in the case where a residual quantity is sequentially detected, the shift of a detected value of a developer residual quantity may cause erroneous detection.
It is an object of the present invention to provide an image forming apparatus for prevention of cyclic density unevenness.
It is a further object of the invention to provide an image forming apparatus, in which an image unevenness caused by a difference between a frequency of a charging voltage and a frequency of a developing voltage is made unnoticeable.
It is a still further object of the invention to provide an image forming apparatus, in which a peripheral speed of an image bearing member can be set to be large.
It is a further object of the invention to provide an image forming apparatus, in which a frequency of a charging voltage and a frequency of a developing voltage are made substantially close to each other.
It is an object of the invention to provide an image forming apparatus, in which detection of a developer residual quantity is stabilized.
Further object and feature of the invention will be made further apparent when reading the following detailed description with reference to the accompanied drawings.